In total, 104 immigrants have died at federal detention centers since 2003.

The deaths revealed Monday were thought to have occurred between 2004 and 2007. The causes were not released.

"Medical care has been the chief concern," says Kevin Keenan, executive director for the ACLU of San Diego and Imperial Counties. "There are no enforceable standards for medical care or anything else inside these facilities – there are guidelines but no way to enforce them."

While the Obama administration has announced plans to overhaul the 32,000-bed detention system that is spread out over 350 county jails, state prisons, and privately run facilities, it has rejected implementing legally enforceable standards at the centers.

Instead, the government will begin moving noncriminal detainees to smaller, less prison-like facilities and ensure that medical care meets federal guidelines, the administration says. Obama also said he'll place federal authorities at the country's largest detention centers to oversee detainee care.

The rapid growth in immigrant detention populations – which is about 30,000, according to Amnesty International – followed tougher post-9/11 immigration policy ordered by the Bush administration.

Immigration advocates say the rise in detainee populations led to neglect and poor conditions inside facilities.

In a March 2009 report on the detention centers, Amnesty International recommended that the administration should use detention as a last resort when determining an immigrant's status.

"Alternatives to detention programs have been shown to be effective and significantly less expensive than holding people in immigration detention in the United States. While the average cost of detaining an immigrant is $95 per person/per day, a study of supervised release conducted by the Vera Institute in New York yielded a 91 percent appearance rate at an estimated cost of just $12 per person per day," according to the Amnesty International report.

In a statement on Monday, ICE Assistant Secretary John Morton said that the revelations of the unreported deaths underline the need for reforms. "This highlights why I am passionate about the need for more direct federal oversight of ICE's detention system and the critical importance of the detention reforms I announced two weeks ago," he said.

As part of the government's reform plan, ICE is creating the Office of Detention Policy and Planning that will be tasked with redesigning the immigrant detention system.

Critics also say that news of the unreported deaths shows a lack of transparency within the detention system.

"For too long, the system of detaining immigration detainees has been devoid of transparency and accountability. This focuses us to question even further whether there are still more deaths that somehow have gone unaccounted for," said David Shapiro, staff attorney for the ACLU's National Prison Project, in a statement.

The Obama administration is coming under increasing pressure from immigrant rights groups pushing for a much more comprehensive revision of American immigration policy. The president has said he will begin working on a bill this year but will not take on what promises to be a controversial process until next year.

ICE enacted standards for its detention facilities in 2000 that groups such as the National Immigration Law Center (NILC) say are acceptable but are often ignored because they are not legally binding.

"What we have seen is that when detainees have urgent medical needs, their pleas for help will go unanswered," says Karen Tumlin, a staff attorney for NILC. "We are hoping that the reforms that are being put in place by the administration will start to change that."